Conventionally in pneumatic tires for use in winter as represented by, for example, studded tires, a tread pattern that includes a plurality of grooves extending in the tire circumferential direction and the tire width direction is provided in the tread portion, in order to ensure traction (driving performance) on snow. It is required that these pneumatic tires have traction on snow, and also have no reduction in braking and driving performance on dry road surfaces with no snow.
It is known that by reducing the percentage of groove area in the tread portion, specifically reducing the area of the grooves as a percentage of the ground contact area in the ground contact patch, and increasing the ground contact area, the adhesion friction force is increased, and the performance of the pneumatic tire on ice is increased. On the other hand, it is known that by increasing the groove area, the drainage characteristics are improved by the tread pattern when the tire is rolling on a wet road surface with a water film.
There are pneumatic tires in which grooves are cyclically formed with a predetermined interval in the tire circumferential direction, and by offsetting their phases on a first side and a second side in the tire width direction with respect to the tire center line, the pattern noise is improved (for example, see Japanese Patent No. 4381787B).
Also, there are pneumatic tires in which a center rib is provided in the center of the tread portion by forming a plurality of grooves in the circumferential direction, and the occurrence of pattern noise is reduced while maintaining drainage characteristics (for example, see Japanese Unexamined Patent Application Publication No. H10-264612A).
Also, there are pneumatic radial tires in which a plurality of inclined grooves is provided at intervals in the circumferential direction inclined from the tire center line side towards the shoulder portion, and, with a structure in which the end portions of the inclined grooves on the center line side are connected to adjacent other inclined grooves, heel and toe wear is reduced (for example, see Japanese Unexamined Patent Application Publication No. H8-142613A).
However, if the ground contact area is increased and the adhesion friction force is raised in order to improve the performance on ice of a pneumatic tire, the drainage characteristics are reduced by the reduction in groove area, and the wet performance (turning performance and braking and driving performance on wet road surfaces) is reduced. Also, the shear force in the snow column (snow column shear force) created by compressing snow into the grooves is reduced, and the performance on snow (turning performance and braking performance on snow) is reduced.
On the other hand, when the groove area is increased in order to increase the drainage characteristics and the snow column shear force, the ground contact area is reduced and the adhesion friction force is reduced, so the performance on ice (turning performance and braking and driving performance on ice) is reduced.